1. Technical Field
The present invention relates to an exhaust gas treatment method and device including a diesel oxidation catalyst and a filter for collecting particulate matter in an exhaust gas passage of an internal combustion engine, particularly a diesel engine which is applied to a drive engine for, e.g., a forklift or the like.
2. Description of the Related Art
In emission control for a diesel engine, a reduction in particulate matter (PM, hereinafter referred to as “PM”) is as important as a reduction in NOx. As effective means for removing the PM from exhaust gas, there is known PM collection means which uses a filter called a DPF filter (Diesel Particulate Filter).
In a DPF filter, when it is in an operation state with a low exhaust gas temperature, the PM is continuously accumulated in the DPF so that it is necessary to perform forced reconditioning in which the exhaust gas temperature is forcibly increased and the PM is thereby burnt and removed.
The forced reconditioning step is described on the basis of FIGS. 11 and 12. In an exhaust gas passage on the upstream side of the DPF filter, there is provided a diesel oxidation catalyst (DOC).
As shown in FIG. 11, fuel is injected to a combustion cylinder by main injection, and the fuel injected by the main injection is combusted to generate an engine output. After the main injection, the fuel is injected by early post-injection. As shown in FIG. 12, the early post-injection is performed at the start point of the forced reconditioning step t1. The fuel injected by the early post-injection is combusted in a high temperature atmosphere in the combustion cylinder, and the diesel oxidation catalyst is heated to an activation temperature (e.g., 250° C. or more) by heat generated by the combustion.
Next, before the bottom dead center (the point t2 in FIG. 12), the fuel is further injected by late post-injection. The fuel injected by the late post-injection is oxidized by the catalysis of the activated diesel oxidation catalyst, and the temperature of exhaust gas is increased to, e.g., 600° C. or more by reaction heat generated in the oxidation. By increasing the temperature of the exhaust gas to 600° C. or more, the PM collected in the DPF filter is burnt and removed from the DPF filter.
Patent Document 1 discloses an exhaust gas purification device in which a diesel oxidation catalyst and a DPF filter device are provided in an exhaust gas passage of a diesel engine, PM in exhaust gas is collected by the DPF filter device, and the collected PM is subjected to forced reconditioning by the above-described means.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-29092
In such exhaust gas treatment device, as shown in FIG. 12, at the stage of increasing the temperature of the diesel oxidation catalyst, there are cases where a large quantity of unburned HC components are released from a muffler outlet until the diesel oxidation catalyst is heated to the activation temperature or more after the fuel is injected by the early post-injection. That is, when the diesel engine is in a low temperature environment or in a low load operation, the exhaust gas temperature is low.
In such case, a problem arises that it takes time for the diesel oxidation catalyst to be heated to the activation temperature so that a large quantity of unburned HC components are released from the muffler outlet during that period. In particular, when an operator manually performs the forced reconditioning with the diesel engine set in an idling state, the exhaust gas temperature is low so that a large quantity of unburned HC components are released.